(a) Field of the invention
The present invention relates to a substrate for an amorphous silicon photoreceptor for electrophotography.
(b) Description of the prior art
There has been developed, of late, a technique of forming a non-crystalline silicon film containing hydrogen atoms (hereinafter to be called briefly a-Si film) on a substrate by decomposing silane gas by a glow discharge and relying on the plasma CVD (Chemical Vapor Deposition) technique. This a-Si film has been put to practice as a semiconductor material which allows the control of its conductivity type and carrier density for the manufacture, at a low cost, of various semiconductor devices of a relatively large area such as solar batteries and thin-film transistors. Also, very recently, the level of technique of this field has made a progress to such a stage that a-Si films having a high resistivity can be obtained with good reproducibility. Therefore, an a-Si film having a high resistivity and being deposited on top of a metal substrate such as aluminum chip has been attracting the interest of those concerned as the material of a photoreceptor for electrophotography which is able to exhibit an excellent property including photoreceptability and mechanical strength. Thus, extensive research and development of such a-Si as a material replacing conventional photoreceptive materials such as selenium (Se) are under way.
An example of the apparatus for the manufacture of such conventional a-Si photoreceptor for electrophotography is shown in FIG. 1. Reference numeral 1 represents a reaction chamber, and this reaction chamber is coupled to an air evacuator 2 for evacuating the interior of the chamber to produce substantial vacuum. A substrate 3 for a photoreceptor is set within this reaction chamber 1. This substrate 3, however, requires to possess an electro-conductivity, so that aluminum (Al) or an Al alloy is used in general as the material thereof. The substrate is provided often in the form of a cylinder in view of the consideration that the substrate 3 is incorporated in a copying machine and like devices. The substrate 3 is arranged to be rotatable within the reaction chamber 1 through a rotator means 4, and moreover arrangement is provided so that the substrate 3 can be subjected to an appropriate temperature at the time of formation of an a-Si film by means of an electric heater 5 provided within the cylindrical substrate 3 and connected to an external power supply 6.
Within the reaction chamber 1, there is provided a cylindrical electrode 7 surrounding the abovesaid cylindrical substrate 3. This electrode 7 is provided with a plurality of gas ejection orifices 8 formed through the wall thereof. These orifices are connected to a gas supply means 9 provided externally of the reaction chamber 1 to be supplied with a material gas such as SiH.sub.4 and other material gases so that the gas ejects into the interior of the electrode 7 under pressure through these orifices 8. A radio frequency electric power is supplied to the electrode 7 from a radio frequency power supply 10 which is connected to this electrode 7 to develop a glow discharge between the electrode 7 and the substrate 3 at an appropriate substrate temperature and under an appropriate gas pressure. As a result, SiH.sub.4 gas and other starting material gases which are supplied into the reaction chamber from the gas supply means 9 are decomposed by the glow discharge, so that a-Si containing silicon hydride is deposited on the surface of the substrate 3. In order to obtain an a-Si film having a high resistivity, such a technique as to include certain volumes of N.sub.2 gas and B.sub.2 H.sub.6 gas into the SiH.sub.4 gas is adopted.
Now, the thickness of an a-Si film which is required for a photoreceptor for electrophotography is said to be in the range of 5 to 50 .mu.m, preferably 10 to 30 .mu.m. With respect to the basic physical property, the layer structure, the layer composition of an a-Si layer itself and also to the manufacturing method of the a-Si layer, there have been and are being made various researches and developments. However, it is the present state of art that hardly any study is being made with respect to the effect, on the property of the photoreceptor, of the a-Si layer serving both as the supporting member and also as the electroconductive material for the substrate of the photoreceptor.
As the material of the substrate of a photoreceptor for electrophotography, metals are desirable because the substrate is required in general to have an electroconductivity. Also, owing to the fact that the formation of an a-Si layer as a film to be provided on top of the substrate is performed while heating the interior of the reaction chamber, so that the substrate requires to be free from being deformed by the application of heat. Furthermore, the substrate is required to be good in workability, i.e. must be easily processed, for the convenience when it is incorporated or mounted in a copying machine, a printer or like devices, and also it is required to have a substantially high mechanical strength, a light weight and a long service life. Not only that, but also the substrate is required to have the property of not giving any adverse effect on the image which is to be obtained. On the basis of these requirements, such metals as aluminum (Al) or aluminum alloys are widely adopted as the material to constitute the substrate of a photoreceptor. This substrate is obtained by first relying on either the extrusion technique or the drawing technique to provide a raw cylindrical structure, and then it is subjected to surface grinding or abrading. In the step prior to the deposition of an a-Si film onto this substrate, it is usual to subject the surface of the substrate to mirror grinding and fat-removing cleaning steps.
The present inventor has discovered that the abovesaid various items of the property of a photoreceptor are markedly affected depending on the quality and property of the substrate employed, and has proposed, in Japanese Patent Application No. Sho 58-135957 Specification, specific conditions concerning the quality and property of alumium alloys for use as the constituting material of the substrate of a photoreceptor. However, as a result of the subsequent detailed experiments conducted by the present inventor, it has been found that, even when the quality and property of the substrate employed are not changed, the crystal grains which are present in the surface of the substrate metal could vary in size due to the difference in the manufacturing methods as well as the processing techniques of the aluminum or aluminum alloy, and that such variation in size of the crystal grains would greatly affect the quality of the electrophotographic image which is to be obtained.